ARTESIAN WATERS IN THE ARID REGION. 607 



impervious strata occur, the water seeps out as 

 springs at their contact. The Llano Estacado, 

 or great Staked Plain of Texas and New Mex- 

 ico, is the largest of all the American mesas in 

 area. Its geological structure is practically that 

 shown in Fig. 3, consisting of a pervious surface 

 formation, averaging three hundred feet, rest- 

 ing upon a foundation of impervious clays and 

 other rock. The upper formation readily im- 

 bibes all the surface rainfall ; hence the region 

 is void of running streams. 



Throughout this large area, once considered 

 hopelessly void of water, good non-flowing wells 

 are now everywhere obtained by boring to the 

 lower depths of the saturated, sponge-like sur- 

 face formation, while springs occasionally break 

 out at the margin of the plains where the two 

 formations are in contact. 



While water-bearing strata should always 

 be porous, and usually are but slightly if at all 

 consolidated, the degree of consolidation has but 

 little bearing upon the retaining function of im- 

 pervious strata. Soft clay shale is practically 

 as impervious as hard slate. In the West many 

 people discredit the possibility of artesian water 

 in many favorable localities, because of the ab- 

 sence of consolidated strata which they suppose 

 are necessary to constitute the impervious stra- 

 tum above the one containing the water. In 

 fact, the less consolidated the rocks of a region 

 are, the more favorable are the artesian condi- 

 tions ; and, inasmuch as the older formations of 

 the earth are more consolidated, metamorphosed, 

 and disturbed by greater tilting, faults, and 

 folds, they are least favorable for the occur- 

 rence of artesian water. Upon the other hand, 

 the later formations present the opposite and 

 more favorable conditions, and with few excep- 

 tions the great artesian wells of the world are 

 found in them. These later rocks play an im- 

 portant part in the geology of the arid region. 



The adjoining section from the Gulf of Mex- 

 ico to the Rocky Mountains, from Galveston to 

 Las Vegas, New Mexico, illustrates some of the 

 principles herein set forth. From the coast to 

 the ninety-seventh meridian is a large series (a) 



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